The present invention relates to a control device, particularly for hydraulically operated hoisting equipment used in raising and lowering loads. An electrically controllable throttle valve is connected to a return line of the hoisting equipment. A pressure scale is provided with at least one blocking position and one control position, and operates in conjunction with an unblocking mechanism.
A generic hydraulic control device for hydraulically operated hoisting equipment is disclosed in DE 44 23 644 C2. The disclosed hydraulic control device is especially provided for lifting masts of fork lift trucks, and has a feed pump connected to a reservoir containing hydraulic fluid and a control piston connected to a return line acting as a pressure scale. The return line is connected to the reservoir. The control piston has an intake connected in the direction of pump delivery upstream from a return valve to a feed line connecting the pump to the hoisting equipment, and is connected by a control line to a connecting line between the pump and the return valve in such a way that the control piston performs the function of a delivery pressure scale in the direction of lowering and the function of a return pressure scale in the direction of hoisting. The control piston is subjected to the load applied by a spring associated with a pretensioning spring acting in the opposite direction. The spring force determines switchover of one control piston as an open pressure scale. When at rest, the spring is not subjected to external influence, and assumes a passage position. As a closed scale, and when at rest the spring has no external force applied to it, and assumes a blocking position. The control system is in the form of a two-way current regulator when the hoisting device moves in the lowering direction and in the form of a three-way current regulator when the device moves in the hoisting direction. The pressure difference in the area of the return valve is evaluated for the purpose of shifting the control piston from its function as delivery pressure scale to the function of the return pressure scale. In addition, a metering orifice designed as throttle valve is mounted between the control piston and the hoisting mechanism. The throttle valve itself is in the form of a two-way valve.
High requirements are set today for the lowering function of forklift trucks. Thus, in addition to the so-called xe2x80x9clift densityxe2x80x9d, a load-independent limit is to be imposed on maximum speed; high lowering speeds are also to be possible when the lift fork is empty, along with sensitive metering of the lowering speed itself. The lowering function in modern forklifts is often performed by a seat valve with a constant opening behavior and a constant volume current regulator, or by a slide valve with delivery pressure scale. It is increasingly possible for seat or slide valve to be operated in proportion by electric means.
In the case of these electrically operated systems, standard EN 1175, Part 1 or 2 (safety of industrial trucksxe2x80x94electric requirements, general requirements for industrial trucks powered by an electric battery or internal combustion engine), requires that it be possible in any event to stop load movement if an error occurs.
An obvious possibility of meeting this requirement would to use an additional series-connected valve. However, this would distinctly reduce the lowering speed when the lift fork is empty, which is not desirable.
If, when a load is lowered by the hydraulic control device referred to above, as disclosed in DE 44 23 644 C2, the delivery pressure scale is kept in its open position, and the return line produces a fluid-conducting connection between the hoisting mechanism and the fluid reservoir in the form of the tank. In this instance, a blocking or switching unit is inserted into the return line upstream from the pressure scale. However, if a failure should occur in the associated blocking unit during the lowering process, the fluid-conducting path is essentially connected up to a throttle point. Also, a hoisting mechanism under load, in particular, may unintentionally move downward, causing considerably safety risks. Thus, the disclosed solution does not comply with Standard EN 1175-1 and 2.
DE 196 22 763 A1 discloses a valve system in the housing in which a spring-loaded valve element is mounted in the direction of closing. The valve element in question controls flow through a fluid channel whose intake may be connected to a fixed displacement pump and whose outlet may be connected to a tank. A control chamber on the side of the valve element is not subjected to pump pressure. A throttle is mounted between the fluid channel and the control chamber. A switching valve connects the control chamber to the tank when the valve is in its idle position, and interrupts this connection when it is in the operating position. An engaging/disengaging thrust bearing for the spring, when the pilot valve is in the operating position, increases the pretensioning of the spring relative to the rest position of the valve. To simplify the valve layout, the valve body and the slide gate of the pilot valve are mounted in a common bore, with a spring being guided between the valve body and the slide gate. The slide gate of the pilot valve serves as thrust bearing for the spring, at least as long as the connection between the control chamber and the tank has been interrupted. With such configuration, the valve layout may be used especially as a relief valve for a consuming device fed by a fixed displacement pump, such as a device in the form of a conventional forklift truck. The disclosed hydraulic valve layout permits the operating states xe2x80x9cslow liftxe2x80x9d and xe2x80x9cfast lift,xe2x80x9d as well as xe2x80x9cslow loweringxe2x80x9d and xe2x80x9cfast lowering,xe2x80x9d in addition to a xe2x80x9cstopxe2x80x9d operating state for the lift fork. While this disclosed solution permits high lowering speeds for an empty lift fork, the associated blocking unit may also experience failure during the lowering process, such as one in the form of stoppage. The disadvantage already described is that in particular a hoisting device (lift fork) under load may move downward unintentionally, resulting in considerable safety hazards.
Objects of the present invention are to provide a control device for hydraulically operated hoisting systems, which is compact, can be made at low production, permits a high lowering speed in normal operation with an empty lift fork, and improves safety so that even in the event of malfunction, an unintentional lowering of the hoisting device with or without load cannot occur.
The foregoing objects are attained by the present invention by the pressure scale, when in its normal position, blocking the return line. The pressure scale assumes its normal position during lowering as a result of triggering of the unblocking mechanism or device. The pressure scale may in the event of failure of the throttle be moved into its blocking position by the unblocking device. The throttle valve assumes its blocking position in the event of failure of the pressure scale. The load retaining function proper is performed by two series-connected hydraulic actuators which may be triggered individually by electric means. Both a hydraulic and an electric redundancy of the load retention function are thereby provided. As a result of the redundant design, the lowering movement is automatically halted, even in the event of failure of one of the two electric or of one of the two hydraulic actuators of the pressure scale and throttle valve.
The throttle valve is closed in the normal position, and is triggered electrically, preferably by way of a proportional pressure control valve. The pressure scale is designed as a delivery pressure scale so that it remains closed in the normal position and assumes the open control position only when the lowering function is actuated. The pressure scale piston is retained in the blocked position by a spring in the process. Unblocking of the piston occurs only as a result of triggering of an unblocking mechanism by way of a directional control valve in the event of presence of a lowering signal, and the pressure scale may perform its control function. If then, for example, the piston of the throttle valve remains in the open position in the event of an error after cutoff of the lowering signal, the movement of the load is then stopped by closing of the pressure scale (failsafe position). Since an additional series-connected valve may be eliminated from the control device of the present invention, the present invention may be applied cost effectively. In addition, the lowering speed during lowering with no load is not needlessly reduced by an additional flow resistance.
The requirement set by Standard EN 1175, Part 1 or Part 2, is also met by the redundant safety. Also, the movement of a load may in any case be halted in the event of an error.
In a preferred embodiment of the control device of the present invention, the unblocking mechanism is an unblocking cylinder with an energy storage element. The storage element is preferably in the form of a pressure spring tending to keep the pressure scale in its blocked position. A pressure supply acts by way of a emergency device with its fluid pressure in the direction opposite the force of the unblocking mechanism. The unblocking mechanism is accordingly capable of displacing the pressure scale in the direction of its blocked position and of retaining it reliably in this blocked position. The unblocking cylinder and the pressure scale preferably are coupled to each other mechanically by an actuating piston of the unblocking cylinder.
In another preferred embodiment of the control device of the present invention, a control line is connected to the return line between the hoisting mechanism and the throttle. This control line forwards the hydraulic pressure present as a control signal. The hydraulic pressure present between throttle valve and pressure scale is forwarded to the pressure scale by means of a tapping line as another control signal acting in the opposite direction. A current control function is thereby achieved.
Other objects, advantages and salient features of the present invention will become apparent from the following detailed description, which taken in conjunction with the annexed drawings, discloses a preferred embodiment of the present invention.